Cutting inserts are utilized in a variety of applications, including use in milling tools and reaming tools. A milling tool may be used for cutting threads, and is then commonly referred to as a thread mill. A reaming tool is used for reaming a hole in a workpiece. The relationship between a thread mill and a reaming tool is that each tool rotates during the cutting operation, and may include several cutting inserts thereon. Numerous differences otherwise exist, however, between a thread mill and a reaming tool.
Thread milling tools may be generally categorized as tools which are indexable or non-indexable. In either case, however, one or more inserts extend radially outward from a tool holder, with each insert including a plurality of cutting teeth positioned parallel or slightly tapered to the tool holder axis for cutting threads on a workpiece. The thread mill tool cutter thus exerts a torsional and radial force on the insert during the cutting operation. One or more cutting inserts on a reamer tool holder may enlarge holes and/or finish a drilled hole to required tolerances. The reamer tool holder rotates while moving along its axis of rotation and the insert, which includes the cutting surface, thus moves axially for reaming the hole. The reamer tool holder thus exerts an axial force on the insert during the reaming operation.
Cutting inserts on some adjustable reamers are secured within the reamer holder by a single screw and dowel assembly. Since the force encountered by a reamer on a workpiece is along the axis of the reamer, the mechanism for securing an insert in place on a reamer is markedly different than securing a threading insert on a milling tool. The functional distinction between reamers and thread milling tools is critical to stabilizing the cutting insert on the tool holder.
Numerous prior art patents are directed specifically to thread mills having multiple cutting inserts. U.S. Pat. No. 4,531,863 to Smith discloses a thread milling device with rotatable thread cutting inserts that are secured against four locating surfaces by two set screws angularly disposed through the cutting insert.
U.S. Pat. No. 4,360,297 to Weber also discloses a thread cutting insert and tool holder. The rotatable insert is secured to the tool holder by a clamp member that is connected to the tool holder and fits within a notch or depression on the insert for securing the insert against four adjacent walls of the tool holder.
U.S. Pat. No. 4,409,868 to Huddle et al. discloses a thread cutting tool having a multi-toothed first thread chaser secured between two adjacent walls by a top clamp and intermediate chip breaker. Also disclosed is a rotatable single tooth thread chaser insert secured within a pocket on the tool holder by both a locking screw passing through a central aperture in the insert into the threaded bore, and a top clamp secured to the tool holder by a clamp screw.
U.S. Pat. No. 4,575,888 to Muren discloses a thread cutting insert having a central aperture for mounting the insert to an indexable tool holder.
U.S. Pat. No. 5,098,232 to Benson discloses a thread milling tool having an insert which may be secured in a pocket of the thread milling tool by a screw extending through a hole in the insert and into a threaded hole in the bottom of the pocket.
E.P. No. 0062621 discloses a thread cutting tool having an indexable insert and shim plate supported against complimentary support surfaces of a recess in the thread cutting tool and secured jointly by a clamp consisting of two bores passing through the insert and shim plate into the thread cutting tool.
U.S. Pat. No. 4,913,604 to Zaengerle discloses a thread milling tool and trapezoidal milling plate secured against four surfaces of the thread milling tool by a countersunk clamping bolt passing through the center of the trapezoidal milling plate.
Numerous disadvantages are presented by the prior art relating to inserts that incorporate a plurality of cutting surfaces thereon, so that the insert can be rotated with respect to the holder when one cutting surface becomes worn, thus precluding the necessity of replacing the insert. Inserts with multiple cutting surfaces are typically secured to a holder by a single clamp or screw in order that the face of the cutting insert may be easily rotated to reveal new cutting edges with minimal delay. Consequently, the stability of the multi-cutting face insert during thread cutting operations is sacrificed. Conversely, once an insert with a single cutting face becomes worn, it must be replaced. The time required to replace the insert and securely remount the new insert on a tool holder is thus a disadvantage of tools with single cutting face inserts.
Disadvantages also exist when a single cutting face insert is secured to the tool holder by a single screw passing through a central axis perpendicular to the insert. For instance, stress may develops in the threaded connection between the screw, insert and tool holder during milling operations, thus reducing stability between the insert and tool holder and reducing the quality of the thread cutting operation.
Multiple screws have been used to secure an insert directly to the tool holder, as evidenced by U.S. Pat. No. 4,531,863 and E.P. No. 0062621, although significant disadvantages remain with these thread mills. Despite the fact that two or more screws are used, the force used to secure the insert to the tool holder is not consistently applied to the insert along the longitudinal axis of the cutting insert. Moreover, the inability to precisely position the cutting insert on the tool holder is increased by securing the insert with one or more screws. Finally, the time required to replace a cutting insert and reliably mount a new insert on a thread mill holder is a significant disadvantage of these prior art tools.
The disadvantages of the prior art are overcome by the present invention. An improved thread mill having multiple thread cutting inserts mounted thereon is hereafter disclosed which overcomes these and other difficulties experienced by prior art tools.